In recent years, a RAID (Redundant Array of Inexpensive Disks) using HDDs (Hard Disk Drive) has been widely used as a typical mass-storage system. RAID combines two or more inexpensive HDDs to perform distributive recording of data having redundancy given, thereby enabling a rapid and reliable storage system at low cost.
A magnetic head records data on a ferromagnetic layer of a magnetic disk in a HDD. There may be many factors influential to a life of HDD. HDD has a disk-like storage part which could have defective portions to be randomly distributed on the disk. Therefore, every location for recording data has the same probability of loosing the data. For this reason, trouble or failure could occur equally in all locations on the disk of HDD and does not depend seriously on the locations for recording data. HDDs included in RAID also have no trouble or failure depending on the locations for dispersively recording data inside the HDDs.
On the other hand, a semiconductor storage unit using semiconductor nonvolatile memories is developing rapidly. The semiconductor storage unit stores data in the semiconductor memories mounted on a mounting board. Unlike the disk of HDD, the semiconductor memories are not necessarily mounted on the substrate with physical symmetry. Therefore, a difference arises in the probability of defects in association with the physical locations of the semiconductor memories on the mounting board. For this reason, it is not enough to simply give redundancy to data for dispersive recording in consideration of a recovery of the data.
Moreover, semiconductor memories have limits of writing frequencies thereof. The limits demand devised equalization of erasing frequencies over the semiconductor memories. For example, it is disclosed that RAID includes two or more flash-memory modules of which erasing frequencies are equalized over the modules. This enables it to extend a service life of the storage system including RAID by the equalization of erasing-frequency over two or more modules included in RAID in order to avoid the erasing-frequency limit peculiar to a semiconductor memory.